Improve Project File Tree test coverage

Closes #224 (closed)

A user on HN highlighted that our Web file tree viewing experience is performing badly. Investigation has found this is due to the Projects::RefsController#logs_tree.json.

MR updates test to include this as well as move to a new path within our gitlabhq project that has many more files. In this case it's app/assets/images/emoji which has around 1.7k files.

█ Results summary

* Environment:                10k
* Environment Version:        12.10.0-pre `71c834efe31`
* Option:                     60s_200rps
* Date:                       2020-04-16
* Run Time:                   2m 8.35s (Start: 14:21:41 UTC, End: 14:23:49 UTC)
* GPT Version:                v1.2.6

❯ Overall Results Score: 70.63%

NAME                            | RPS   | RPS RESULT          | TTFB AVG  | TTFB P90            | REQ STATUS     | RESULT
--------------------------------|-------|---------------------|-----------|---------------------|----------------|-----------------
api_v4_projects_repository_tree | 200/s | 195.5/s (>160.00/s) | 78.09ms   | 96.64ms (<500ms)    | 100.00% (>95%) | Passed
web_project_files               | 20/s  | 8.7/s (>16.00/s)    | 1818.33ms | 2450.08ms (<4000ms) | 100.00% (>95%) | FAILED³